Free written scope consultation - allowances, timeline, and change-order rules on paper before a single trade is scheduled
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AfterFast, Reliable Home Additions & ADU Across Millbrae
Adding square footage to a Millbrae home usually means one of three things: a second-story addition on a hillside lot, a garage conversion ADU on the flat east side, or a detached unit in the backyard. Each one carries its own permitting layer, from FAA airspace filings to geotechnical review to Bay mud compaction requirements. A typical home addition in Millbrae runs $45,000 to $130,000, and a new ADU starts around $90,000. We’ve been pulling permits and filing the right forms here long enough to know which one your project triggers before the first hammer swings. Call (877) 610-1318 for a free estimate and we’ll tell you exactly what you’re looking at.
Why Barrett & Sons Construction Is Millbrae’s Preferred Home Additions & ADU Company
Tom Barrett founded this company in 1998, and his sons run the crews. That’s not marketing language. When you call, a live person answers. When we schedule a walkthrough in Millbrae, the person who shows up is the person who will be on your job. We serve homeowners from the 94030 zip code who have been burned by outfits that quote one number on the phone and hand them a different bill at the door. Our answer to that is simple: every scope item, every allowance, and every price change arrives as a written change order before work proceeds. No exceptions.
Ask anyone on your street, or look up the license number yourself. Tom publishes his name, his address, and his license number. He lives close enough that if something isn’t right, he wants to hear about it. We back every job with a written 90-Day Done Right Promise. If it’s not done right, one call brings Tom back.
Millbrae is not an easy city to build in. The FAA airspace rules, the hillside geotech flags, the older housing stock built between the late 1940s and mid-1960s. None of that scares us. It’s what we plan around every day. Our Home Additions & ADU team files FAA Form 7460-1 as standard practice when a crane or scaffolding is involved. Most contractors don’t even know the form exists until the crane gets turned away. We file it before the building permit is pulled.
Our Home Additions & ADU Services in Millbrae
ADU Design & Permitting
Millbrae sits directly adjacent to SFO’s active runways and falls within FAA Part 77 airspace obstruction zones. That means any ADU project involving cranes, scaffolding, or structures that penetrate the imaginary surfaces above the city must file FAA Form 7460-1 before breaking ground. This is a federal permitting layer virtually no other residential Bay Area city faces, and it can add weeks if you don’t know it’s coming. We file it as part of our design and permitting package, not as a surprise after plans are drawn. A new detached ADU in Millbrae typically runs $90,000 to $220,000, and getting the permitting right from day one is what keeps that number from creeping up.
Garage Conversion ADU
Converting a garage on the flat east side of Millbrae means dealing with Bay mud foundation conditions. The existing slab was likely poured decades ago without the compaction current codes require. That doesn’t mean you can’t convert it. It means we need to evaluate the slab before we promise you a number. A garage conversion ADU in Millbrae typically runs $30,000 to $80,000, and the foundation work is often the largest variable. We write the allowance into the contract so you know the range before we start, and any adjustment comes as a written change order.
New Detached ADU
Backyard space in Millbrae is limited, especially on the western hillside parcels where retaining walls and soils reports are part of the deal. A detached ADU on a sloped lot requires engineered retaining walls, geotechnical review, and often a crane for panelized or modular construction. That crane triggers the FAA notification. We handle the whole chain, from soils report to FAA filing to final inspection sign-off. On the east side, where lots are flatter but Bay mud is the concern, we focus on foundation design and drainage so your unit doesn’t settle unevenly.
Junior ADU & Interior Conversion
A junior ADU inside an existing Millbrae home is the most budget-friendly path to rental income or multigenerational living. These conversions typically stay inside the existing footprint, which means fewer site complications. But opening walls in a 1950s rancher often turns up knob-and-tube wiring that can’t support new circuits. We plan for that in the estimate. The permit inspector works for you, not for us, and we’d rather he sign off on a job that’s safe than a job that was cheap.
Second Story Additions
Adding a second story is the single most common way Millbrae homeowners add real square footage, and it’s also the most technically demanding. The west-side hillside lots routinely get flagged for geotechnical review by the City of Millbrae’s building department due to mapped landslide hazard zones. The original framing in these post-WWII homes was never designed for the seismic loads a second story adds. We sister load-bearing walls, upgrade connectors, and reroute outdated wiring before the inspector ever walks through. That’s not optional. It’s what a real addition costs in Millbrae.
Sunrooms & Enclosed Patios
Millbrae’s position on the lower Peninsula traps marine-layer moisture and concentrates winter rainfall against the coastal hills. That means any ground-level sunroom or enclosed patio needs proper site drainage, waterproofing details, and grading, or you’ll be fighting water intrusion every winter. We build sunrooms with the same drainage standards we’d use on a full addition, because the marine layer doesn’t care how much you saved on the slab.
What happens when you call
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A real person answersNo phone trees - you reach a local pro.
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You get an upfront price rangeHonest numbers before anyone is dispatched.
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A background-checked tech heads outLicensed & insured, dispatched right away.
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You approve before work beginsNothing starts until you say go.
Trusted Brands We Service in Millbrae
We install materials you can look up, not mystery brands off a van. On Millbrae additions, we regularly use GAF roofing and moisture barriers, Sterling plumbing fixtures, and Schluter waterproofing systems in kitchens and baths. These are brands with published specs and warranties. When a homeowner in Millbrae asks us why we picked a certain material, we can hand them the spec sheet and the warranty. That matters more on a hillside parcel than it does on a flat lot, because the weather hits harder and the stakes are higher. We keep common parts and materials staged for Millbrae jobs so a supply run doesn’t stall your project for a week.
Common Home Additions & ADU Problems We See in Millbrae Homes
- Knob-and-tube wiring behind 1950s drywall. Opening walls for an addition in a Millbrae rancher regularly exposes wiring that can’t legally support new circuits. We reroute it and write the cost into a change order before proceeding.
- FAA notification missed on crane days. We’ve seen crane deliveries turned away because Form 7460-1 wasn’t filed. The pour or framing day is lost, and the crew still gets paid. We file before the permit is pulled.
- Undersized framing on west-side hillside lots. Millbrae homes built between the late 1940s and mid-1960s were framed for lighter loads. Adding a second story without sistering the walls and upgrading Simpson Strong-Tie connectors is a liability, not a shortcut.
- Bay mud settlement on east-side slabs. Flat east-side parcels near the bayfront sit on compressible soils. A slab designed without enough compaction will settle differentially, cracking the new unit’s walls and floor. We compact and test before we pour.
Pricing for Home Additions & ADU in Millbrae, CA
What does it actually cost to add space in Millbrae? Here are honest ranges, not phone quotes.
| Project Type | Typical Range in Millbrae |
|---|---|
| Home Addition | $45,000 - $130,000 |
| ADU Construction | $90,000 - $220,000 |
| Garage Conversion ADU | $30,000 - $80,000 |
These ranges reflect real Millbrae conditions: FAA filings, geotech review on the west side, Bay mud on the east side, and older framing that needs upgrades once walls are open. The biggest cost variable is what we find when we open things up. That’s why we price unknowns as written allowances in the contract, and every adjustment comes as a written change order before work proceeds. An allowance that isn’t on paper is a surprise waiting for a date. Call (877) 610-1318 for a free estimate, and you’ll get a line-item written quote before you sign anything.
We Also Serve Cities Near Millbrae
Our crews work throughout the upper Peninsula. If you’re in San Bruno, Burlingame, Hillsborough, or South San Francisco, we serve those communities too. The permitting rules change from one city to the next, and we know the differences. Visit our Home Additions & ADU in Millbrae page for local details, or call us to talk about your project wherever you are.
Serving Millbrae, CA - Our Local Coverage Area
We’re based in the Millbrae area and know this community well. Use the map below to see our service coverage - if you’re nearby, we can almost certainly help.
FAQs - Home Additions & ADU in Millbrae
Yes. Millbrae is the only city on the Peninsula where residential additions or ADUs requiring a crane, scaffolding, or structural height above the FAA Part 77 imaginary surfaces must file FAA Form 7460-1 before pulling a building permit. This federal layer does not exist in Burlingame, San Bruno, or South San Francisco. If the form isn’t filed, the crane gets turned away and you lose the day. We file it as part of our permitting package, before the building permit is pulled. Call (877) 610-1318 and we’ll confirm whether your project triggers the filing.
Because the western hillside parcels have mapped landslide hazard zones and expansive clay soils. The City of Millbrae’s building department flags projects on these lots for geotechnical review as standard practice. That review may require a soils report, engineered retaining walls, and revised foundation drawings. We build that time and cost into the estimate up front, not after the plans come back flagged. Call (877) 610-1318 for an estimate that accounts for it from day one.
We reroute it. Knob-and-tube wiring can’t legally support the new circuits an addition requires, and leaving it in place would fail inspection. We write the reroute into a change order before we touch it, with the exact cost and scope. No surprise invoice at the end. On a recent hillside lot off Millbrae Avenue, we opened up a 1950s rancher for a second-story addition and found exactly this. Our crew sistered the load-bearing walls, our electrician rerouted the wiring, and we wrote the upgraded Simpson Strong-Tie connectors into a change order before the inspector sign-off. Call (877) 610-1318 to talk through your home’s specific wiring age and what it likely means for your project.
We write them as allowances in the contract. The biggest unknown on the east side is the condition of the existing slab, which may sit on Bay mud without adequate compaction. We include an allowance range for slab evaluation and potential over-excavation or compaction work. If the allowance isn’t used, you don’t pay it. If it is, you approve the change order in writing before we proceed. A garage conversion ADU in Millbrae typically runs $30,000 to $80,000, with the foundation being the largest variable. Call (877) 610-1318 for a written estimate that maps those unknowns out.
No. The inspector reviews the work you’re doing, not the entire house. But certain conditions, like knob-and-tube wiring in the areas we open or undersized framing in the load path, must be corrected as part of the permitted scope. We flag those before we start, not after the inspector walks through. The permit inspector works for you, not for us, and we’d rather he sign off on a safe job than a cheap one. Call (877) 610-1318 for a free walkthrough and we’ll tell you what your specific home will trigger.
Written by Tom Barrett, Owner at Barrett & Sons Construction, serving Millbrae since 1998.
Ready to talk about your Millbrae addition or ADU? Call (877) 610-1318 today. We’ll walk your property, answer your questions straight, and hand you a written line-item estimate before you commit to anything. Free estimates, written change orders, and a 90-Day Done Right Promise on every job.